This proposal is aimed at furthering our understanding of the interactions between sperm and egg proteins, and the evolutionary forces driving reproductive protein evolution. The work employs the abalone system, which provides one of the most extensively studied sperm-egg interaction systems and which is amenable to biochemical and experimental studies difficult in other taxa. Two specific aims are proposed: (1) identify and characterize the egg plasma-membrane bound receptor of the abalone sperm protein sp18; and (2) characterize polymorphism and divergence of a candidate sp18 receptor already identified (ZPD) within and among abalone species, and test for co-evolution with sp18. Specific Aim 1 utilizes biochemical approaches (immunoprecipitation and affinity columns) and mass spectrometry to identify proteins from fractionated egg components which bind with high specificity to sp18. This work will test for evidence of direct functional interactions between sp18 and egg molecules. Specific Aim 2 examines polymorphism at a candidate sp18 receptor locus (ZPD) identified in genomic screens for egg coat proteins, as well as divergence at this locus among California abalone species. Patterns of polymorphism and sequence divergence for ZPD will be tested for evidence of positive selection as well as coevolution with sp18. Because these signatures are predicted for the sp18 receptor, they provide an independent and parallel means of distinguishing among candidate egg receptors (including ZPD and any others which may be identified under Specific Aim 1). Together these research aims represent a significant advancement in our understanding of the molecular interactions between sperm and egg, and contribute to a broader understanding of reproductive biology including human reproduction. [unreadable] [unreadable] The interactions between sperm and egg molecules during fertilization have proven elusive in studies of human reproduction. The model organism used in this study (abalone, a marine gastropod) shares a common molecular basis with vertebrates (including humans and other mammals), exhibits a common pattern of rapid divergence of sperm and egg proteins, and allows for experimental approaches not possible in other animals. Thus the proposed work addresses general themes in reproductive biology, and may shed light on the molecular basis of sperm and egg interactions of relevance to human reproduction. [unreadable] [unreadable] [unreadable]